In the following discussion, it is largely assumed that the medical device is provided for use on or in a human or animal body, and accordingly has at least one interface via which the device either outputs signals at a predetermined body location or records them therefrom. The identified interface does not have to be a body-device interface, but rather may also relate to (for example) a terminal of the medical device which may be connected to further measuring or active devices.
Medical devices, in particular electrostimulators (and especially cardiac pacemakers) are currently receiving ever greater attention because of their life-saving and life-sustaining properties. Many medical devices may take over complex physical functions of patients, or support them by appropriately conditioned electrical signals which charge/energize particular body parts. Furthermore, medical devices are often used for the purpose of recording electrical signals from a patient's body and using them for diagnosis or for therapy methods after corresponding processing. Modern medical devices frequently have therapeutic and diagnostic functions, as is the case for modern cardiac pacemakers, for example, which monitor the cardiac muscle activity using special measuring devices adapted for this purpose, and which may also control the muscle activity.
Medical devices of this type share the feature that they have a body-device interface through which they are in contact with a human or animal body for electrical signal transmission. Comparable body-device interfaces are also provided, for example, in external and implantable defibrillators; stimulation configurations for stimulating the auditory nerve; or other implantable measuring and transmission configurations for intracorporeal detection and analysis and/or external transmission of measured values of physiological variables.
If multiple medical devices of this type are used simultaneously in a patient, the danger exists of mutual electrical influence of the functional capability of the devices, which may possibly cause interference or damage to components contained therein by electrical overvoltages. This may occur, for example, if an electrical voltage of a first medical device is coupled via the body of the patient into a body-device interface of a second medical device. If the coupled-in voltage signals result from relatively high voltages applied to the body, as may be output by defibrillators or HF-surgical devices (for example), a second medical device's internal electronic components may be destroyed if the components to which high voltage is applied do not have a suitable dielectric strength.
To avoid internal overvoltages by external coupling, many electrotechnical devices use overvoltage protection circuits, which allow overvoltages coupled into an interface to be reduced to a predetermined minimal value and thus electronic components to be protected from exceedingly high voltage differences and the resulting destruction from electrical voltage breakdown. Overvoltage protection circuits of this type have already proven themselves for decades in many applications in electronic circuitry technology.
A typical overvoltage protection circuit in an input stage of a device, for example, includes a series circuit made of a resistor and a Zener diode, in which the coupled-in overvoltage energy is converted into thermal energy.
A further example of an overvoltage protection circuit, which includes a voltage limiting element provided between two line inputs of an electronic circuit to be protected, and also a current limiting device connected in series to the voltage limiting element, is disclosed in U.S. Pat. No. 5,751,531 A. The current limiting device may be implemented as a MOS transistor, whose gate and source terminals are short-circuited with one another and is only interconnected via its source and drain terminals. The voltage limiting element is typically implemented as a Zener diode.
A similar overvoltage protection circuit, which includes two 60 V Zener diodes connected to one another in a reverse direction, and connected between an input terminal and a ground terminal, is disclosed in U.S. Pat. No. 4,661,979. Furthermore, this overvoltage protection circuit includes two diodes connected to one another in the reverse direction inside an integrated circuit, which are provided between the two inputs of the integrated circuit and an internal ground terminal, and a diode connected in the forward direction, which is connected between the internal ground terminal and a voltage supply.
Known overvoltage protection circuits described above allow the implementation of a short-circuit upon application of an overvoltage signal, via which the high voltages and energies, which possibly result in damage to further electronic components, may be dissipated.
However, if a defibrillator is used on a body provided with further body-device interfaces of a medical device, the implementation of a shunt circuit in the medical devices to be protected would have the result that the defibrillation energy is not completely active on the heart of the patient, but rather would be dissipated from the body of the patient via the short-circuit and would thus be lost for the defibrillation. This is particularly the case if the body-device interface is situated in proximity to the body region to which the defibrillation energy is applied, as is the case with stimulators and cardiac pacemakers.
It is therefore desirable for the energy output during defibrillation via the defibrillator electrodes to act substantially completely on the body of the patient and not be dissipated unused via a body-device interface of a further medical device.